The primary functions of a drilling fluid or mud are: to carry chips and cuttings produced by drilling to the surface; to lubricate and cool the drill bit and drill string; to form a filter cake which obstructs filtrate invasion in the formation; to maintain the walls of the borehole; to control formation pressures and prevent lost returns; to suspend cuttings during rig shutdowns; and to protect the formation for later successful completion and production.
Useful drilling fluids or muds must maintain rheological and viscosity properties under normal operation conditions. Also, the drilling fluids or muds must be able to suspend cuttings and weighting materials upon stopping of circulation of the drilling fluid. It is desirable to have drilling fluids or muds which maintain thixotropy and rheology even with increased solids.
U.S. Pat. Nos. 2,938,016; 2,980,653; 3,085,994; 3,342,787; 3,418,292; 3,451,979; 3,536,461; 3,558,570; 3,702,300; and 3,723,375 relate to carboxy-containing interpolymers, methods of making the carboxy-containing interpolymers, and esters and nitrogen-containing esters derived therefrom.
U.S. Pat. No. 2,977,334 relates to derivatives of ethylene-maleic anhydride copolymers. These polymers are useful as coating resins, synthetic varnish for drying oils, additive for mineral or synthetic oils, paper and textile sizing agents, laminating, casting and potting resins, adhesive agents, waterproofing agents, film-forming polymers and plasticizers.
U.S. Pat. No. 3,738,934 issued to Broning et al. relates to oil-base drilling fluid compositions, additives and filter cakes which are effective under high temperature pressures. The fluid is formed by dispersing particles of an oil and water insoluble vinyl toluene-acrylate copolymer resin in a water-in-oil emulsion drilling fluid.
U.S. Pat. No. 4,526,950 relates to polymers of alpha-olefins and alpha, beta-unsaturated carboxylic acids or derivatives and methods of making these polymers.
U.S. Pat. No. 4,566,978 relates to high temperature drilling fluids. The drilling fluid comprises a water-base, clay suspended in the base and a hydrolyzed terpolymer of maleic anhydride, styrene and a third monomer selected from a acrylamide, methacrylamide, acrylic acid or methacrylic acid.